In the art of flexographic printing, and in particular the use of an impression cylinder, a web passes between a printing cylinder and an impression cylinder. The impression cylinder is typically a smooth metal cylinder, and it supports the web against the printing cylinder. The printing cylinder is also typically a smooth metal cylinder, and carries one or more rubber or elastomeric printing plates. These plates pick up rapid drying fluid ink from an anilox (ink metering) roll, and deposit it on surface of the web in contact with the printing cylinder. The anilox roll in turn transfers ink from a fountain roll, or an open or closed doctor blade system, in communication with a source of ink.
Flexographic techniques are discussed in Flexography Principles and Practices, second edition, copyright 1970, published by the Flexographic Technical Association, Inc.; and Flexographic Printing, copyright 1958, published by F.E. Boughton.
For certain market applications, it is sometimes desirable to print the surfaces of a web, especially a thermoplastic web in rollstock form, or the outer surfaces of a flattened tubular web, with a continuous, uniform background color. For example, to create a smoked appearance for certain food packaging applications, a flattened tubing will be printed on the outer surfaces of the tubing using a flexographic process as described above. The printing cylinder will in this case be covered with a continuous rubber or elastomeric covering rather than discrete, spaced rubber or elastomeric printing plates. The web to be printed, e.g. with a dark brown ink, will be drawn between the impression cylinder and the printing cylinder. The ink will then be deposited on the web in the manner described above.
It is desirable from an aesthetic standpoint that the ink cover the entire outer surfaces of the tube, i.e. extend across the entire transverse width, as well as length, of the tube. To assure this complete transverse coverage, the printing cylinder is inked along a greater transverse width than the width of the web to be printed. This assures printing of the lateral extremities of the web. However, this also results in the undesirable build up of excess ink on the impression cylinder, along and beyond the two lateral edges of the web. It is costly, difficult, and time consuming to remove this build up of excess ink from the impression cylinder.
An alternative to avoid this build up of ink is to ink the printing cylinder along a transverse width somewhat less than the width of the web to be printed. However, this approach results in a rollstock or flattened tube which is printed along a central portion thereof, but clear (in the case of clear thermoplastic rollstock or tubing) along its lateral (transverse) edges. This results in a less attractive web for some of the end uses for which the web is intended.
The inventor has discovered a method, described herein, for obtaining full transverse ink coverage of a web without significant ink build up on the impression cylinder.
The term "web" is used herein to include rollstock or flattened tubing, to be printed on one or both sides, and includes paper; thermoplastic films, sheets, or laminates; and other materials susceptible to coloration or printing in a flexographic process or a process as described herein.
The term "printed" is used herein to indicate the deposition of an ink on all or part of the surface of a web. This includes the deposition of a single ink in a uniform, continuous manner on the web to effect a uniformly colored web (extending along all or some of the entire transverse width of the web) e.g. as a colored "coating" to the web. This also includes the use of printing plates to impart singly or in sequence a monochromatic or multicolored graphic design on the web, e.g. as a trade dress or design e.g. to identify a particular brand.
It is an object of the invention to obtain full transverse coverage of a web without significant ink build up on the impression cylinder.
The foregoing object is accomplished by the present invention which is summarized below.